Capacitance reduction for backside power rail device

ABSTRACT

The present disclosure describes a method to form a backside power rail (BPR) semiconductor device with an air gap. The method includes forming a fin structure on a first side of a substrate, forming a source/drain (S/D) region adjacent to the fin structure, forming a first S/D contact structure on the first side of the substrate and in contact with the S/D region, and forming a capping structure on the first S/D contact structure. The method further includes removing a portion of the first S/D contact structure through the capping structure to form an air gap and forming a second S/D contact structure on a second side of the substrate and in contact with the S/D region. The second side is opposite to the first side.

BACKGROUND

With advances in semiconductor technology, there has been increasingdemand for higher storage capacity, faster processing systems, higherperformance, and lower costs. To meet these demands, the semiconductorindustry continues to scale down the dimensions of semiconductordevices, such as metal oxide semiconductor field effect transistors(MOSFETs), including planar MOSFETs and fin field effect transistors(finFETs). Such scaling down has increased the complexity ofsemiconductor manufacturing processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures.

FIGS. 1A-1C illustrate an isometric view and various cross-sectionalviews of a backside power rail (BPR) semiconductor device withcapacitance reduction using an air gap, in accordance with someembodiments.

FIGS. 2A-2D illustrate cross-sectional views of various BPRsemiconductor devices with capacitance reduction using an air gap, inaccordance with some embodiments.

FIG. 3 is a flow diagram of a method for fabricating a BPR semiconductordevice with capacitance reduction using an air gap, in accordance withsome embodiments.

FIGS. 4-13 illustrate cross-sectional views of a BPR semiconductordevice with capacitance reduction using an air gap at various stages ofits fabrication process, in accordance with some embodiments.

Illustrative embodiments will now be described with reference to theaccompanying drawings. In the drawings, like reference numeralsgenerally indicate identical, functionally similar, and/or structurallysimilar elements.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over a second feature in the description that followsmay include embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Asused herein, the formation of a first feature on a second feature meansthe first feature is formed in direct contact with the second feature.In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition does not in itselfdictate a relationship between the various embodiments and/orconfigurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” “exemplary,” etc., indicatethat the embodiment described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases do not necessarily refer to the same embodiment. Further,when a particular feature, structure or characteristic is described inconnection with an embodiment, it would be within the knowledge of oneskilled in the art to effect such feature, structure or characteristicin connection with other embodiments whether or not explicitlydescribed.

It is to be understood that the phraseology or terminology herein is forthe purpose of description and not of limitation, such that theterminology or phraseology of the present specification is to beinterpreted by those skilled in relevant art(s) in light of theteachings herein.

In some embodiments, the terms “about” and “substantially” can indicatea value of a given quantity that varies within 5% of the value (e.g.,±1%, ±2%, ±3%, ±4%, ±5% of the value). These values are merely examplesand are not intended to be limiting. The terms “about” and“substantially” can refer to a percentage of the values as interpretedby those skilled in relevant art(s) in light of the teachings herein.

With increasing demand for lower power consumption, higher performance,and smaller area (collectively referred to as “PPA”) of semiconductordevices, backside power rails (BPR) can be implemented in semiconductordevices to reduce the device area and the metal interconnect length,thus reducing parasitic capacitances and parasitic resistances andimproving device performance. For example, backside power rails canimprove performance of power delivery networks (PDNs) for advancedtechnology nodes. BPR semiconductor devices can have front-side S/Dcontact structures and interconnect structures at the front-side andbackside S/D contact structures and interconnect structures at thebackside to reduce device area, parasitic capacitance and resistance,and improve device performance. For example, front-side S/D contactstructures and interconnect structures can connect a drain region of aBPR semiconductor device to front-side power rails. Backside S/D contactstructures and interconnect structures can connect a source region ofthe BPR semiconductor device to backside power rails. The source regionof the BPR semiconductor device can be connected to a dummy front-sideS/D contact structure though it's connected to backside S/D contactstructures. The dummy front-side S/D contact structure is not connectedto front-side interconnect structures or front-side power rails, but itcan introduce parasitic capacitance between the gate structure and thedummy front-side S/D contact structure of the BPR semiconductor device.The parasitic capacitance can degrade the device performance of the BPRsemiconductor device.

Various embodiments in the present disclosure provide methods forforming a BPR semiconductor device with capacitance reduction using anair gap. According to some embodiments, the BPR semiconductor device canhave first and second S/D regions adjacent to opposite ends of a finstructure on a front side of a substrate. First and second S/D contactstructures each having a metal contact and a silicide layer can beformed in contact with the first and second S/D regions, respectively.The first S/D contact structure can be connected to front-side powerrails by a first interconnect structure through a first cappingstructure on the first S/D contact structure. The second S/D contactstructure can be removed through an opening in a second cappingstructure over the second S/D region. After removal of the second S/Dcontact structure, an air gap can form between the second cappingstructure and the second S/D region. In some embodiments, the metalcontact of the second S/D contact structure can be removed through theopening. In some embodiments, both the metal contact and the silicidelayer of the second S/D contact structure can be removed through theopening. In some embodiments, a seal dielectric structure can be formedin the opening to seal the air gap between the second capping structureand the second S/D region. In some embodiments, a seal dielectric layercan be formed in the air gap during the formation of the seal dielectricstructure in the opening. According to some embodiments, the air gap canreduce the parasitic capacitance between the second S/D region and gatestructures of the BPR semiconductor device. In some embodiments, thedevice performance of the BPR semiconductor device can be improved byabout 3% to about 5.5% due to capacitance reduction using the air gap.

FIG. 1A illustrates an isometric view of a BPR semiconductor device 100with capacitance reduction using an air gap 126, in accordance with someembodiments. BPR semiconductor device 100 can include a FET 102. A firstinterconnect structure 114 (also referred to as “front-side interconnectstructure 114”) can connect a first S/D region of FET 102 to front-sidepower rails 105. A second interconnect structure 104 (also referred toas “backside interconnect structure 104”) can connect a second S/Dregion of FET 102 to backside power rails 103. FIG. 1B illustrates across-sectional view of BPR semiconductor device 100 along line B-B inFIG. 1A, in accordance with some embodiments. FIG. 1C illustrates anenlarged view of region C in FIG. 1B, according to some embodiments. Insome embodiments, FIGS. 1A-1C show a portion of an IC layout where thefin structures and the gate structures can be similar or different fromthe one shown in FIGS. 1A-1C.

Referring to FIGS. 1A-1C, BPR semiconductor device 100 can include FET102, front-side and backside interconnect structures 114 and 104connected to front-side power rails 105 and backside power rails 103,respectively. FET 102 can further include a fin structure 108, first andsecond S/D regions 110A and 110B, gate structures 112, gate spacers 116,and inner spacer structures 127.

In some embodiments, FET 102 can be a p-type finFET (PFET) or an n-typefinFET (NFET). The term “p-type” can be associated with a structure,layer, and/or region doped with p-type dopants, such as boron. The term“n-type” can be associated with a structure, layer, and/or region dopedwith n-type dopants, such as phosphorus. Though FIGS. 1A-1C show onefinFET, BPR semiconductor device 100 can have any number of finFETs. Inaddition, BPR semiconductor device 100 can be incorporated into anintegrated circuit through the use of other structural components, suchas conductive vias, conductive lines, dielectric layers, and passivationlayers, that are not shown for simplicity.

FET 102 can be formed on a first side (e.g., front-side) of substrate406, as shown in FIGS. 4-10. In some embodiments, substrate 406 caninclude a semiconductor material, such as silicon (Si). In someembodiments, substrate 406 can include a silicon-on-insulator (SOI)substrate (e.g., SOI wafer). In some embodiments, substrate 406 caninclude (i) an elementary semiconductor, such as germanium (Ge); (ii) acompound semiconductor including silicon carbide (SiC), silicon arsenide(SiAs), gallium arsenide (GaAs), gallium phosphide (GaP), indiumphosphide (InP), indium arsenide (InAs), indium antimonide (InSb),and/or a III-V semiconductor material; (iii) an alloy semiconductorincluding silicon germanium (SiGe), silicon germanium carbide (SiGeC),germanium stannum (GeSn), silicon germanium stannum (SiGeSn), galliumarsenic phosphide (GaAsP), gallium indium phosphide (GaInP), galliumindium arsenide (GaInAs), gallium indium arsenic phosphide (GaInAsP),aluminum indium arsenide (AlInAs), and/or aluminum gallium arsenide(AlGaAs); (iv) a silicon germanium (SiGe)-on insulator structure(SiGeOI); (v) germanium-on-insulator (GeOI) structure; or (vi) acombination thereof. Further, substrate 406 can be doped depending ondesign requirements (e.g., p-type substrate or n-type substrate). Insome embodiments, substrate 406 can be doped with p-type dopants (e.g.,boron, indium, aluminum, or gallium) or n-type dopants (e.g., phosphorusor arsenic).

As shown in FIG. 1B, BPR semiconductor device 100 can include finstructure 108 extending along an X-axis and through FET 102. Finstructure 108 can include a stack of semiconductor layers 122, which canbe nanosheets or nanowires. Each of semiconductor layers 122 can form achannel region underlying gate structures 112 of FET 102. Embodiments ofthe fin structures disclosed herein may be patterned by any suitablemethod. For example, the fin structures may be patterned using one ormore photolithography processes, including double-patterning ormulti-patterning processes. Double-patterning or multi-patterningprocesses can combine photolithography and self-aligned processes,forming patterns that have, for example, pitches smaller than what isotherwise obtainable using a single, direct photolithography process.For example, a sacrificial layer is formed over a substrate andpatterned using a photolithography process. Spacers are formed alongsidethe patterned sacrificial layer using a self-aligned process. Thesacrificial layer is then removed, and the remaining spacers may then beused to pattern the fin structures.

In some embodiments, semiconductor layers 122 can include semiconductormaterials similar to or different from substrate 406. In someembodiments, each of semiconductor layers 122 can include Si without anysubstantial amount of or can include silicon germanium (SiGe) with Ge ina range from about 5 atomic percent to about 50 atomic percent Ge withany remaining atomic percent being Si. The semiconductor materials ofsemiconductor layers 122 can be undoped or can be in-situ doped duringits epitaxial growth process using: (i) p-type dopants, such as boron,indium, and gallium; and/or (ii) n-type dopants, such as phosphorus andarsenic. Though three layers of semiconductor layers 122 for FET 102 areshown in FIG. 1B, FET 102 can have any number of semiconductor layers122.

Referring to FIGS. 1A-1C, first and second S/D regions 110A and 110B canbe disposed adjacent to opposite ends of fin structure 108. In someembodiments, first and second S/D regions 110A and 110B can have anygeometric shape, such as a polygon, an ellipsis, and a circle. First andsecond S/D regions 110A and 110B can include an epitaxially-grownsemiconductor material. In some embodiments, the epitaxially-grownsemiconductor material is the same material as substrate 406. In someembodiments, the epitaxially-grown semiconductor material includes adifferent material from substrate 406. In some embodiments, theepitaxially-grown semiconductor material for first and second S/Dregions 110A and 110B can be the same as or different from each other.The epitaxially-grown semiconductor material can include: (i) asemiconductor material, such as germanium and silicon; (ii) a compoundsemiconductor material, such as gallium arsenide and aluminum galliumarsenide; or (iii) a semiconductor alloy, such as silicon germanium andgallium arsenide phosphide.

In some embodiments, first and second S/D regions 110A and 110B can ben-type or p-type. In some embodiments, n-type first and second S/Dregions 110A and 110B can include Si and can be in-situ doped during anepitaxial growth process using n-type dopants, such as phosphorus andarsenic. In some embodiments, n-type first and second S/D regions 110Aand 110B can have multiple n-type epitaxial fin sub-regions that candiffer from each other based on, for example, doping concentrationand/or epitaxial growth process conditions. In some embodiments, p-typefirst and second S/D regions 110A and 110B can include SiGe and can bein-situ doped during an epitaxial growth process using p-type dopants,such as boron, indium, and gallium. In some embodiments, p-type firstand second S/D regions 110A and 110B can have multiple sub-regions thatcan include SiGe and can differ from each other based on, for example,doping concentration, epitaxial growth process conditions, and/orrelative concentration of Ge with respect to Si. For example, as shownin FIG. 1B, first S/D region 110A can include first epitaxial sub-region110A-1 and second epitaxial sub-region 110A-2. In some embodiments,first epitaxial sub-region 110A-1 can have a width along an X-axisranging from about 10 nm to about 30 nm and a thickness along a Z-axisranging from about 5 nm to about 10 nm. In some embodiments, secondepitaxial sub-region 110A-2 can have a width along an X-axis rangingfrom about 10 nm to about 30 nm and a thickness along a Z-axis rangingfrom about 30 nm to about 50 nm.

Referring to FIGS. 1A and 1B, fin structure 108 can be current-carryingstructures for FET 102. Channel regions of FET 102 can be formed inportions of their respective fin structure 108 underlying gatestructures 112. First and second S/D regions 110A and 110B can functionas source/drain regions of FET 102.

Referring to FIGS. 1A-1C, gate structures 112 can be multi-layeredstructures and can be wrapped around semiconductor layers 122 of finstructure 108. In some embodiments, each of semiconductor layers 122 offin structure 108 can be wrapped around by one or more layers of gatestructures 112, respectively, and gate structures 112 can be referred toas “gate-all-around (GAA) structures” and FET 102 can be referred to as“GAA FET” or “GAA finFET.”

Gate structures 112 can include a gate dielectric layer and a gateelectrode wrapped around semiconductor layers 122. The gate dielectriclayer can be wrapped around each of semiconductor layers 122, and thuselectrically isolate semiconductor layers 122 from each other and fromthe conductive gate electrode to prevent shorting between gatestructures 112 and semiconductor layers 122 during operation of FET 102.In some embodiments, the gate dielectric layer can include aninterfacial layer and a high-k layer. The term “high-k” can refer to ahigh dielectric constant. In the field of semiconductor devicestructures and manufacturing processes, high-k can refer to a dielectricconstant that is greater than the dielectric constant of SiO₂ (e.g.,greater than about 3.9). In some embodiments, the interfacial layer caninclude silicon oxide. In some embodiments, the high-k layer can includehafnium oxide (HfO₂), zirconium oxide (ZrO₂), or any suitable high-kdielectric materials. In some embodiments, the gate electrode caninclude a gate barrier layer, a gate work function layer, and a gatemetal fill layer. Each of semiconductor layers 122 can be wrapped aroundby one or more of the gate barrier layer, the gate work function layer,and the gate metal fill layer. In some embodiments, the gate electrodecan include titanium (Ti), tantalum (Ta), titanium nitride (TiN),tantalum nitride (TaN), aluminum (Al), copper (Cu), tungsten (W), cobalt(Co), or other suitable conductive materials.

Referring to FIGS. 1B and 1C, gate spacers 116 can be disposed alongsidewalls of gate structures 112, and inner spacer structures 127 can bedisposed between portions of gate structures 112 and first and secondS/D regions 110A and 110B. Each of gate spacers 116 and inner spacerstructures 127 can include a dielectric material, such as silicon oxide(SiO_(x)), silicon oxynitride (SiO_(y)N), silicon nitride (SiN_(x)),silicon oxycarbide (SiOC), silicon carbonitride (SiCN), siliconoxynitricarbide (SiOCN), and a combination thereof. In some embodiments,each of gate spacers 116 and inner spacer structures 127 can include asingle layer or multiple layers of insulating materials. In someembodiments, gate spacers 116 can isolate gate structures 112 andadjacent S/D contact structures. Inner spacer structures 127 can isolategate structures 112 and first and second S/D regions 110A and 110B.

Referring to FIGS. 1A-1C, BPR semiconductor device 100 can furtherinclude a front-side S/D contact structure 132A, a dummy silicide layer128B, an air gap 126, a first etch stop layer (ESL) 124, a gate cappingstructure 134, first and second capping structures 136A and 136B, a sealdielectric structure 142, front-side interconnect structure 114, asecond ESL 138, a front-side interlayer dielectric (ILD) layer 140, aliner 144, a backside ILD layer 146, a backside contact structure 148, abarrier layer 150, and backside interconnect structure 104. As shown inFIGS. 1A-1C, front-side S/D contact structure 132A and front-sideinterconnect structure 114 can connect first S/D region 110A tofront-side power rails 105. Backside contact structure 148 and backsideinterconnect structure 104 can connect second S/D region 110B tobackside power rails 103. In some embodiments, front-side power rails105 and backside power rails can include power supply lines and groundlines for BPR semiconductor device 100.

Front-side S/D contact structure 132A can include a silicide layer 128Aand a metal contact 130A. Silicide layer 128A can include metalsilicides and can reduce the contact resistance between metal contact130A and first S/D region 110A of FET 102. Examples of metals used forforming the metal silicides on n-type S/D regions can include titanium(Ti), chromium (Cr), tantalum (Ta), molybdenum (Mo), zirconium (Zr),hafnium (Hf), scandium (Sc), yttrium (Y), holmium (Ho), terbium (Tb),gadolinium (Gd), lutetium (Lu), dysprosium (Dy), erbium (Er), ytterbium(Yb), and other suitable metals. Examples of metals used for forming themetal silicides on p-type S/D regions can include nickel (Ni), cobalt(Co), manganese (Mn), tungsten (W), iron (Fe), rhodium (Rh), palladium(Pd), ruthenium (Ru), platinum (Pt), iridium (Ir), osmium (Os), andother suitable metals. In some embodiments, silicide layer 128A can havea thickness along a Z-axis ranging from about 1 nm to about 10 nm. Insome embodiments, dummy silicide layer 128B can include a same metalsilicide as silicide layer 128A and can have a thickness ranging fromabout 1 nm to about 10 nm.

Metal contact 130A can include metals, such as tungsten (W), ruthenium(Ru), cobalt (Co), copper (Cu), titanium (Ti), titanium nitride (TiN),tantalum (Ta), tantalum nitride (TaN), molybdenum (Mo), nickel (Ni),metal alloys, or other suitable metals. In some embodiments, metalcontact 130A can have a thickness along a Z-axis ranging from about 10nm to about 50 nm.

As shown in FIGS. 1B and 1C, air gap 126 can be disposed between secondcapping structure 136B and second S/D region 110B. Air gap 126 can beformed by removing the dummy metal contact on second S/D region 110B andcan be filled with air. According to some embodiments, air gap 126,which replaces the metal contact, can reduce an overlapping area betweengate structures 112 and second S/D region 110B, and thus reduce theparasitic capacitance between gate structures 112 and second S/D region110B. The capacitance reduction using the air gap can improve the deviceperformance of BPR semiconductor device 100 by about 3% to about 5.5%.In some embodiments, air gap 126 can have a horizontal dimension 126 w(e.g., width) along an X-axis ranging from about 5 nm to about 30 nm.Air gap 126 can have a vertical dimension 126 h (e.g., height) along aZ-axis ranging from about 5 nm to about 50 nm. In some embodiments, aratio between vertical dimension 126 h to horizontal dimension 126 w canrange from about 0.1 to about 10. If vertical dimension 126 h is lessthan about 5 nm, or the ratio is less than about 0.1, the deviceperformance of BPR semiconductor device 100 may not be improved. Ifvertical dimension 126 h is greater than about 50 nm, or the ratio isgreater than about 10, air gap 126 may not be properly sealed. Inaddition, horizontal dimension 126 w can be limited by a distancebetween adjacent gate structures, and vertical dimension 126 h can belimited by surrounding structures.

Gate capping structure 134 can be disposed on gate structures 112 andconfigured to protect underlying structures and/or layers duringprocessing of BPR semiconductor device 100. For example, gate cappingstructure 134 can act as an etch stop layer during the formation offront-side S/D contact structures on first and second S/D regions 110Aand 110B. Gate capping structure 134 can include one or more insulatingmaterials. In some embodiments, the insulating materials can includesilicon oxide (SiO_(x)), hafnium silicide (HfSi), silicon oxycarbide(SiOC), aluminum oxide (AlO_(x)), zirconium silicide (ZrSi), aluminumoxynitride (AlON), zirconium oxide (ZrO), hafnium oxide (HfO), titaniumoxide (TiO), zirconium aluminum oxide (ZrAlO), zinc oxide (ZnO),tantalum oxide (TaO), lanthanum oxide (LaO), yttrium oxide (YO),tantalum carbonitride (TaCN), silicon nitride (SiN_(x)), siliconoxynitricarbide (SiOCN), silicon (Si), zirconium nitride (ZrN), siliconcarbonitride (SiCN), or other suitable materials. In some embodiments,gate capping structure 134 can have a thickness along a Z-axis rangingfrom about 0 nm to about 50 nm. In some embodiments, gate cappingstructure 134 can have a width along an X-axis ranging from about 5 nmto about 30 nm. In some embodiments, BPR semiconductor device 100 canhave no gate capping structure.

First capping structure 136A can be disposed on front-side S/D contactstructure 132A and second capping structure 136B can be disposed aboveair gap 126. First and second capping structures 136A and 136B can beconfigured to protect adjacent structures (e.g., gate structures 112)during the formation of front-side interconnect structure 114 andformation of seal dielectric structure 142. In some embodiments, firstand second capping structures 136A and 136B can include insulatingmaterials, such as silicon oxide (SiO_(x)), silicon oxycarbide (SiOC),aluminum oxide (AlO_(x)), aluminum oxynitride (AlON), zirconium oxide(ZrO), hafnium oxide (HfO), titanium oxide (TiO), zirconium aluminumoxide (ZrAlO), zinc oxide (ZnO), tantalum oxide (TaO), lanthanum oxide(LaO), yttrium oxide (YO), tantalum carbonitride (TaCN), silicon nitride(SiN_(x)), silicon oxynitricarbide (SiOCN), zirconium nitride (ZrN),silicon carbonitride (SiCN), and other suitable materials. In someembodiments, first and second capping structures 136A and 136B caninclude same insulating materials. In some embodiments, gate cappingstructure 134 can include insulating materials different from first andsecond capping structures 136A and 136B, which can have different etchselectivity and to further protect adjacent structures (e.g., gatestructures 112). The term “etch selectivity” can refer to the ratio ofthe etch rates of two different materials under the same etchingconditions. In some embodiments, the etch selectivity between gatecapping structure 134 and first and second capping structures 136A and136B can range from about 15 to about 20. For example, gate cappingstructure 134 can include silicon nitride, and first and second cappingstructures 136A and 136B can include silicon oxide. In some embodiments,first and second capping structures 136A and 136B can have a thicknessalong a Z-axis ranging from about 0 nm to about 50 nm and a width alongan X-axis ranging from about 5 nm to about 30 nm. In some embodiments,BPR semiconductor device 100 can have no capping structures onfront-side S/D contact structure 132A.

Referring to FIGS. 1A-1C, front-side interconnect structure 114 canconnect to front-side S/D contact structure 132A and extends throughfirst capping structure 136A, second etch stop layer (ESL) 138, andfront-side ILD layer 140. Front-side interconnect structure 114 caninclude metals, such as tungsten (W), ruthenium (Ru), cobalt (Co),copper (Cu), titanium (Ti), titanium nitride (TiN), tantalum (Ta),tantalum nitride (TaN), molybdenum (Mo), nickel (Ni), metal alloys, andother suitable metals. In some embodiments, front-side interconnectstructure 114 can have a thickness along a Z-axis ranging from about 1nm to about 50 nm.

Seal dielectric structure 142 can extend through second cappingstructure 136B, second ESL 138, and front-side ILD layer 140 to seal airgap 126. Seal dielectric structure 142 can include insulating materials,such as silicon oxide (SiO_(x)), silicon oxycarbide (SiOC), aluminumoxide (AlO_(x)), aluminum oxynitride (AlON), zirconium oxide (ZrO),hafnium oxide (HfO), titanium oxide (TiO), zirconium aluminum oxide(ZrAlO), zinc oxide (ZnO), tantalum oxide (TaO), lanthanum oxide (LaO),yttrium oxide (YO), tantalum carbonitride (TaCN), silicon nitride(SiN_(x)), silicon oxynitricarbide (SiOCN), zirconium nitride (ZrN),silicon carbonitride (SiCN), and other suitable materials. In someembodiments, seal dielectric structure 142 can have a thickness along aZ-axis ranging from about 10 nm to about 50 nm and a width along anX-axis ranging from about 5 nm to about 30 nm. In some embodiments, aratio of the width of seal dielectric structure 142 to the width ofsecond capping structure 136B can range from about 0.3 to about 0.6. Ifthe width of seal dielectric structure 142 is less than about 5 nm, orthe ratio is less than about 0.3, the metal contact on second S/D region110B may not be removed and air gap 126 and seal dielectric structure142 may not be formed. If the width of seal dielectric structure 142 isgreater than about 15 nm, or the ratio is greater than about 0.6, sealdielectric structure 142 may not seal air gap 126 and may fill in airgap 126. In some embodiments, seal dielectric structure 142 can have aconcave surface adjacent to air gap 126. The concave surface can be around surface or a triangular surface formed due to the depositionprocess of seal dielectric structure 142.

First ESL 124 can be formed on sidewalls of gate spacers 116 and onfirst and second S/D regions 110A and 110B. First ESL 124 can protectgate structure 112 and/or portions of first and second S/D regions 110Aand 110B that are not in contact with front-side S/D contact structure132A. This protection can be provided, for example, during formation offront-side S/D contact structure 132A. In some embodiments, first ESL124 can include, for example, silicon nitride (SiN_(x)), silicon oxide(SiO_(x)), silicon oxynitride (SiON), silicon carbide (SiC), or acombination thereof. Second ESL 138 can be formed on gate cappingstructure 134 and first and second capping structures 136A and 136B. Insome embodiments, second ESL 138 can include dielectric materials, suchas aluminum oxide, to protect underlying structures during the formationof front-side interconnect structures.

Front-side ILD layer 140 can be disposed on first ESL 124. Front-sideILD layer 140 can include a dielectric material to isolate front-sideinterconnect structure 114 and other interconnect structures. Thedielectric material can be deposited using a deposition method suitablefor flowable dielectric materials (e.g., flowable silicon oxide,flowable silicon nitride, flowable silicon oxynitride, flowable siliconcarbide, or flowable silicon oxycarbide). In some embodiments, thedielectric material can be silicon oxide. In some embodiments,front-side ILD layer 140 can have a thickness along a Z-axis rangingfrom about 10 nm to about 25 nm. Backside ILD layer 146 can be disposedon another side (e.g., backside) of fin structure 108 opposite tofront-side S/D contact structure 132A. Backside ILD layer 146 caninclude a similar dielectric material to front-side ILD layer 140 andcan provide isolation between backside interconnect structure 104 andother backside interconnect structures.

Liner 144 can be disposed between backside ILD layer 146 and first andsecond S/D regions 110A and 110B. In some embodiments, liner 144 canprovide protection to gate structures 112 and first and second S/Dregions 110A and 110B during the formation of backside ILD layer 146.Backside contact structure 148 can include a silicide layer similar toor different from silicide layers 128A and 128B. In some embodiments,backside contact structure 148 can reduce the contact resistance betweenbackside interconnect structure 104 and second S/D region 110B. Barrierlayer 150 can be disposed between backside interconnect structure 104and liner 144. In some embodiments, barrier layer 150 can preventdiffusion of metals from backside interconnect structure 104 to backsideILD layer 146.

In some embodiments, BPR semiconductor device 100 can further includeother structures, such as metal lines, metal vias, and dielectricstructures, to provide connection to and isolation from other parts ofthe IC layout. These structures are not shown in details merely forclarity and ease of description.

FIG. 2A illustrates a cross-sectional view of a BPR semiconductor device200-1 with capacitance reduction using an air gap 226-1, in accordancewith some embodiments. FIG. 2B illustrates an enlarged view of region Bin FIG. 2A, in accordance with some embodiments. FIG. 2C illustrates across-sectional view of a BPR semiconductor device 200-2 withcapacitance reduction using an air gap 226-2, in accordance with someembodiments. FIG. 2D illustrates an enlarged view of region D in FIG.2C, in accordance with some embodiments. Elements in FIGS. 2A-2D withthe same annotations as elements in FIGS. 1A-1C are described above.

As shown in FIGS. 2A-2B, BPR semiconductor device 200-1 can include nodummy silicide layers on second S/D region 110B. Because of the absenceof dummy silicide layers, air gap 226-1 can have increased dimensionsand the parasitic capacitance between gate structures 112 and second S/Dregion 110B can be further reduced. In some embodiments, air gap 226-1can have a horizontal dimension 226-1 w (e.g., width) along an X-axisranging from about 5 nm to about 30 nm. Air gap 226-1 can have avertical dimension 226-1 h (e.g., height) along a Z-axis ranging fromabout 10 nm to about 50 nm. In some embodiments, a ratio betweenvertical dimension 226-1 h to horizontal dimension 226-1 w can rangefrom about 0.1 to about 10.

As shown in FIGS. 2C-2D, BPR semiconductor device 200-2 can have a sealdielectric layer 242 disposed on second S/D region 110B in air gap226-2. Seal dielectric layer 242 can be formed on the surfaces of secondS/D region 110B and first ESL 124 during the deposition of sealdielectric structure 142. In some embodiments, seal dielectric layer 242can have a thicker thickness at bottom corners in air gap 226-2. Becauseforming seal dielectric layer 242, air gap 226-2 can have reduceddimensions and the parasitic capacitance between gate structures 112 andsecond S/D region 110B can increase. In some embodiments, air gap 226-2can have a horizontal dimension 226-2 w (e.g., width) along an X-axisranging from about 5 nm to about 20 nm. Air gap 226-2 can have avertical dimension 226-2 h (e.g., height) along a Z-axis ranging fromabout 5 nm to about 40 nm. In some embodiments, a ratio between verticaldimension 226-1 h to horizontal dimension 226-1 w can range from about0.2 to about 8. In some embodiments, BPR semiconductor device 200-1including no dummy silicide layers can have a seal dielectric layer inair gap 226-1 (not shown) formed during the fabrication processes. Theseal dielectric layer in air gap 226-1 can also reduce dimensions of airgap 226-1 and increase the parasitic capacitance between gate structures112 and second S/D region 110B.

FIG. 3 is a flow diagram of a method 300 for fabricating BPRsemiconductor device 100 with capacitance reduction using air gap 126,in accordance with some embodiments. Method 300 may not be limited toGAA FETs and can be applicable to devices that would benefit fromcapacitance reduction using an air gap, such as planar FETs, fin FETs,etc. Additional fabrication operations may be performed between variousoperations of method 300 and may be omitted merely for clarity and easeof description. Additional processes can be provided before, during,and/or after method 300; one or more of these additional processes arebriefly described herein. Moreover, not all operations may be needed toperform the disclosure provided herein. Additionally, some of theoperations may be performed simultaneously or in a different order thanshown in FIG. 3. In some embodiments, one or more other operations maybe performed in addition to or in place of the presently describedoperations. For example purposes, the operations illustrated in FIG. 3will be described with reference to the example fabrication process forfabricating BPR semiconductor device 100 as illustrated in FIGS. 4-13.FIGS. 4-13 are cross-sectional views of BPR semiconductor device 100.Although FIGS. 4-13 illustrate fabrication processes of BPRsemiconductor device 100 with capacitance reduction using air gap 126 inFET 102, method 300 can be applied to other FETs in BPR semiconductordevice 100, BPR semiconductor devices 200-1 and 200-2, and other BPRsemiconductor devices. Elements in FIGS. 4-13 with the same annotationsas elements in FIGS. 1A-1C are described above.

In referring to FIG. 3, method 300 begins with operation 310 of forminga fin structure on a first side of a substrate. For example, as shown inFIG. 4, fin structure 108 can be formed on a first side 406S1 ofsubstrate 406. In some embodiments, substrate 406 can be Si. Finstructure 108 can include semiconductor layers 122. In some embodiments,semiconductor layers 122 can include Si. Gate structures 112 can wraparound each of semiconductor layers 122. Gate spacers 116 can be formedon sidewalls of gate structures 112 above fin structure 108. Innerspacer structures 127 can be formed adjacent to gate structures 112 andbetween semiconductor layers 122. Gate capping structure 134 can beformed on gate structures 112 to protect gate structures 112.

The formation of fin structure 108 can include epitaxially growingsemiconductor layers having different etch selectivity in an alternatingconfiguration. The alternating semiconductor layers can be removed andinner spacer structures 127 and gate structures 112 can be formedbetween semiconductor layers 122. The formation of fin structure 108 canbe followed by a vertical etch to form openings 452A and 452B adjacentto opposite ends of fin structure 108.

In operation 320 of FIG. 3, first and second source/drain (S/D) regionscan be formed adjacent to opposite ends of the fin structure. Forexample, as shown in FIGS. 5 and 6, first and second S/D regions 110A*and 110B* can be formed in openings 452A and 452B, respectively,adjacent to opposite ends of fin structure 108 on first side 406S1 ofsubstrate 406. In some embodiments, dummy epitaxial layers can be formedin openings 452A and 452B for backside connections to second S/D region110B*.

Prior to forming first and second S/D regions 110A* and 110B*, opening452B can be etched with opening 452A blocked by a mask layer and a firstdummy epitaxial layer 554 can be epitaxially grown in opening 452Bextending into substrate 406. In some embodiments, first dummy epitaxiallayer 554 can have a width 554 w along an X-axis ranging from about 10nm to about 30 nm and a thickness 554 t along a Z-axis ranging fromabout 20 nm to about 50 nm. In some embodiments, first dummy epitaxiallayer 554 can include silicon germanium (SiGe) with Ge in a range fromabout 5 atomic percent to about 15 atomic percent with any remainingatomic percent being Si. In some embodiments, first dummy epitaxiallayer can be replaced by backside interconnect structures in subsequentprocesses.

The formation of first dummy epitaxial layer 554 can be followed byepitaxial growth of second dummy epitaxial layers 110A-0 and 110B-0 inopenings 452A and 452B, respectively. In some embodiments, second dummyepitaxial layers 110A-0 and 110B-0 can include SiGe with Ge in a rangefrom about 20 atomic percent to about 35 atomic percent with anyremaining atomic percent being Si. In some embodiments, second dummyepitaxial layers 110A-0 and 110B-0 can have a width along an X-axisranging from about 10 nm to about 30 nm and a thickness along a Z-axisranging from about 5 nm to about 10 nm. In some embodiments, seconddummy epitaxial layers 110A-0 and 110B-0 can protect first and secondS/D regions 110A and 110B during subsequent processes of substrate 406removal.

First epitaxial sub-regions 110A-1 and 110B-1 and second epitaxialsub-regions 110A-2 and 110B-2 can be epitaxially grown on second dummyepitaxial layers 110A-0 and 110B-0, respectively. In some embodiments,first epitaxial sub-regions 110A-1 and 110B-1 can have a width along anX-axis ranging from about 10 nm to about 30 nm and a thickness along aZ-axis ranging from about 5 nm to about 10 nm. In some embodiments,second epitaxial sub-regions 110A-2 and 110B-2 can have a width along anX-axis ranging from about 10 nm to about 30 nm and a thickness along aZ-axis ranging from about 30 nm to about 50 nm. In some embodiments,first epitaxial sub-regions 110A-1 and 110B-1 and second epitaxialsub-regions 110A-2 and 110B-2 can include SiGe and can be in-situ dopedusing p-type dopants for p-type first and second S/D regions 110A and110B. In some embodiments, first epitaxial sub-regions 110A-1 and 110B-1and second epitaxial sub-regions 110A-2 and 110B-2 can include Si andcan be in-situ doped using n-type dopants for n-type first and secondS/D regions 110A and 110B.

After forming the dummy epitaxial layers and the epitaxial sub-regions,first S/D region 110A can include second dummy epitaxial layer 110A-0,first epitaxial sub-region 110A-1, and second epitaxial sub-region110A-2. Second S/D region 110B can include first dummy epitaxial layer554, second dummy epitaxial layer 110B-0, first epitaxial sub-region110B-1, and second epitaxial sub-region 110B-2.

In operation 330 of FIG. 3, a first S/D contact structure can be formedin contact with the first S/D region, and a second S/D contact structurecan be formed in contact with the second S/D region. For example, asshown in FIG. 7, first S/D contact structure 132A (also referred to as“front-side S/D contact structure 132A”) can be formed in opening 452Ashown in FIG. 6 and in contact with first S/D region 110A*. Second S/Dcontact structure 132B (also referred to as “dummy S/D contact structure132B”) can be formed in opening 452B shown in FIG. 6 and in contact withsecond S/D region 110B*. Prior to forming first and second S/D contactstructures 132A and 132B, first ESL 124 can be formed to protect gatestructure 112 and/or portions of first and second S/D regions 110A and110B that are not in contact with first and second S/D contactstructures 132A and 132B.

The formation of first and second S/D contact structures 132A and 132Bcan include removal of a portion of first ESL 124 on first and secondS/D regions 110A and 110B, formation of silicide layers 128A and 128B,and deposition of metals in openings 452A and 452B followed by achemical mechanical polishing (CMP) process to coplanarize top surfacesof gate capping structure 134 and first and second S/D contactstructures 132A and 132B. First S/D contact structure 132A can includesilicide layer 128A and metal contact 130A and can be connected to aninterconnect structure in subsequent processes. Second S/D contactstructure 132B can include silicide layer 128B and metal contact 130Band may not connect to any interconnect structures. As a result, secondS/D contact structure 132B can be referred to as “dummy S/D contactstructure 132B,” silicide layer 128B can be referred to as “dummysilicide layer 128B,” and metal contact 130B can be referred to as“dummy metal contact 130B.” In some embodiments, silicide layer 128A anddummy silicide layer 128B can include same metal silicides and metalcontact 130A and dummy metal contact 130B can include same metals.

In operation 340 of FIG. 3, a first capping structure can be formed onthe first S/D contact structure and a second capping structure can beformed on the second S/D contact structure. For example, as shown inFIG. 7, first capping structure 136A* can be formed on first S/D contactstructure 132A and second capping structure 136B* can be formed onsecond S/D contact structure 132B. First and second S/D contactstructures 132A and 132B can be etched back. Capping dielectricmaterials can be deposited on first and second S/D contact structures132A and 132B followed by a CMP process to coplanarize top surfaces ofgate capping structure 134 and first and second capping structures 136A*and 136B*. In some embodiments, first and second capping structures136A* and 136B* can have a thickness along a Z-axis ranging from about 0nm to about 50 nm and a width along an X-axis ranging from about 5 nm toabout 30 nm. In some embodiments, BPR semiconductor device 100 can haveno capping structures on front-side S/D contact structure 132A.

In operation 350 of FIG. 3, a portion of the second S/D contactstructure can be removed through the second capping structure to form anair gap. For example, as shown in FIGS. 8 and 9, a portion of second S/Dcontact structure 132B can be removed through second capping structure136B* to form air gap 126. The removal of the portion of second S/Dcontact structure 132B can include forming opening 842 in second cappingstructure 136B*, etching the portion of second S/D contact structure132B, and forming seal dielectric structure 142 in opening 842. Prior toforming opening 842, second ESL 138 and front-side ILD layer 140 can beblanket deposited on top surfaces of gate capping structure 134 andfirst and second capping structures 136A* and 136B*, as shown in FIG. 8.

The blanket depositions of second ESL 138 and front-side ILD layer 140can be followed by forming opening 842 by a patterning process. A masklayer can be deposited on front-side ILD layer 140 to open areas oversecond S/D region 110B for an etching process. Front-side ILD layer 140,second ESL 138, and second capping structure 136B* can be etched to formopening 842. In some embodiments, the etching process can includemultiple steps, in which each step can remove one of front-side ILDlayer 140, second ESL 138, and second capping structure 136B. In someembodiments, the etching process can stop on dummy metal contact 130B.In some embodiments, opening 842 can have a width along an X-axisranging from about 5 nm to about 15 nm.

The formation of opening 842 can be followed by the etching of theportion of second S/D contact structure 132B to form air gap 126. Asshown in FIG. 8, dummy metal contact 130B of second S/D contactstructure 132B can be etched and removed through opening 842. In someembodiments, dummy metal contact 130B can be removed by a wet etchingprocess. In some embodiments, the wet etching process can includeetchants of sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and hydrogenperoxide (H₂O₂) at various concentrations. In some embodiments, the wetetching process can be performed at a temperature ranging from about 25°C. to about 125° C. In some embodiments, the wet etching process canremove dummy metal contact 130B and keep dummy silicide layer 128B, asshown in FIGS. 1A-1C and 8. In some embodiments, the wet etching processcan remove both dummy metal contact 130B and dummy silicide layer 128B,which are shown in FIGS. 2A and 2B.

The etching of the portion of second S/D contact structure 132B to formair gap 126 can be followed by forming seal dielectric structure 142 inopening 842. As shown in FIG. 9, seal dielectric structure 142 can beformed in opening 842 shown in FIG. 8 to seal air gap 126. In someembodiments, seal dielectric structure 142 can be formed by depositing aseal dielectric material in opening 842 by atomic layer deposition (ALD)or other suitable deposition methods. The seal dielectric materials caninclude insulating materials, such as silicon oxide (SiO_(x)), siliconoxycarbide (SiOC), aluminum oxide (AlO_(x)), aluminum oxynitride (AlON),zirconium oxide (ZrO), hafnium oxide (HfO), titanium oxide (TiO),zirconium aluminum oxide (ZrAlO), zinc oxide (ZnO), tantalum oxide(TaO), lanthanum oxide (LaO), yttrium oxide (YO), tantalum carbonitride(TaCN), silicon nitride (SiN_(x)), silicon oxynitricarbide (SiOCN),zirconium nitride (ZrN), silicon carbonitride (SiCN), and other suitablematerials. In some embodiments, the seal dielectric material can bedeposited at a temperature ranging from about 100° C. to about 400° C.In some embodiments, the seal dielectric material can be deposited usingprecursors including silane (SiH₄) and other suitable precursors. Theseal dielectric material can be deposited at a deposition rate rangingfrom about 10 Å/min to about 100 Å/min for about 5 deposition cycles toabout 30 deposition cycles. In some embodiments, the deposition processcan form seal dielectric structure 142 in opening 842, as shown in FIGS.1A-1C and 9. In some embodiments, the seal dielectric material can bedeposited on sidewalls in opening 842 and can merge at the middle ofopening 842, thereby forming a concave surface adjacent to air gap 126.In some embodiments, the deposition process can deposit a layer of theseal dielectric material in air gap 126 before sealing opening 842,therefore forming seal dielectric structure 142 in opening 842 and sealdielectric layer 242 in air gap 126 as shown in FIGS. 2C and 2D. Thedeposition of seal dielectric structure 142 can be followed by a CMPprocess to coplanarize top surfaces of front-side ILD layer 140 and sealdielectric structure 142.

The formation of seal dielectric structure 142 can be followed theformation of front-side interconnect structure 114 and front-side powerrails 105. As shown in FIG. 10, front-side interconnect structure 114can be formed in first capping structure 136A to connect to front-sideS/D contact structure 132A. The formation of front-side interconnectstructure 114 can include a patterning process to form an opening infront-side ILD layer 140, second ESL 138, and first capping structure136A, similar to the patterning process to form opening 842. Metals canbe deposited into the opening to form front-side interconnect structure114.

In operation 360 of FIG. 3, a third contact structure is formed on asecond side of the substrate in contact with the second S/D region. Thesecond side is opposite to the first side. For example, as shown inFIGS. 11-13, third contact structure 148 can be formed on second side(e.g., backside) 406S2 of substrate 406 (also referred to as “backsidecontact structure 148”). Second side 406S2 can be opposite to first side406S1. Prior to forming backside contact structure 148, BPRsemiconductor device 100 can be bonded to a carrier substrate (notshown) on first side 406S1 (e.g., front-side) of substrate 406 forsubsequent fabrication processes on second side 406S2 of substrate 406.

The bonding of BPR semiconductor device 100 to the carrier substrate canbe followed by a removal of substrate 406 by an etching process, asshown in FIG. 11. In some embodiments, the etching process can include adry etching process using etchants including chlorine (Cl₂), boronchloride (BCl₃), and oxygen (O₂). A flow rate of the etchants can rangefrom about 5 sccm to about 200 sccm. The dry etching process can beperformed at a pressure ranging from about 1 mTorr to about 100 mTorrwith a plasma power ranging from about 50 W to about 250 W. The removalof substrate 406 can be followed by removal of second dummy epitaxiallayer 110A-0 as shown in FIG. 11. In some embodiments, first dummyepitaxial layer 554 and second dummy epitaxial layer 110B-0 may not beremoved during the removal of substrate 406 and second dummy epitaxiallayer 110A-0 due to different etch rates. For example, during theremoval of substrate 406 including silicon, first dummy epitaxial layer554 and second dummy epitaxial layer 110A-0 may include silicongermanium having a lower etch rate and may not be removed. During theremoval of second dummy epitaxial layer 110A-0, first dummy epitaxiallayer 554 may include silicon germanium with a different germaniumconcentration and have a different etch rate, thereby protecting seconddummy epitaxial layer 110B-0.

The removal of substrate 406 and second dummy epitaxial layer 110A-0 canbe followed by the deposition of liner 144 and backside ILD layer 146,as shown in FIG. 12. In some embodiments, liner 144 can provideprotection to gate structures 112 and first and second S/D regions 110Aand 110B** during formation of backside ILD layer 140. In someembodiments, backside ILD layer 146 can be deposited using a depositionmethod suitable for flowable dielectric materials. In some embodiments,backside ILD layer 146 can include flowable silicon oxide, flowablesilicon nitride, flowable silicon oxynitride, flowable silicon carbide,or flowable silicon oxycarbide. In some embodiments backside ILD layer146 can isolate backside interconnect structure 104 of FET 102 fromother adjacent devices and structures.

The deposition of liner 144 and backside ILD layer 146 can be followedby the removal of first dummy epitaxial layer 554 and second dummyepitaxial layer 110B-0. For p-type first and second epitaxialsub-regions 110B-1 and 110B-2, the removal of first dummy epitaxiallayer 554 and second dummy epitaxial layer 110B-0 may have an over etchof first epitaxial sub-region 110B-1 and may remove first epitaxialsub-region 110B-1 due to similar etch selectivity between second dummyepitaxial layer 110B-0 and p-type first epitaxial sub-region 110B-1. Forn-type first and second epitaxial sub-regions 110B-1 and 110B-2, theremoval of first dummy epitaxial layer 554 and second dummy epitaxiallayer 110B-0 may stop on first epitaxial sub-region 110B-1 due to higheretch selectivity between second dummy epitaxial layer 110B-0 and n-typefirst epitaxial sub-region 110B-1.

The removal of first dummy epitaxial layer 554 and second dummyepitaxial layer 110B-0 can be followed by the formation of third contactstructure 148 on second side 406S2 (also referred to as “backsidecontact structure 148”), as shown in FIG. 13. In some embodiments,backside contact structure 148 can include a silicide layer formed witha metal silicide the same as or different from silicide layers 128A and128B. In some embodiments, backside contact structure 148 can reduce thecontact resistance between second S/D region 110B and backsideinterconnect structure 104. The formation of backside contact structure148 can be followed by formation of barrier layer 150, backsideinterconnect structure 104, and backside power rails 103.

Various embodiments in the present disclosure provide methods forforming BPR semiconductor devices 100, 200-1, and 200-2 with capacitancereduction using air gaps 126, 226-1, and 226-2, respectively. Accordingto some embodiments, BPR semiconductor device 100 can have first andsecond S/D regions 110A and 110B adjacent to opposite ends of finstructure 108 on front side 406S2 of substrate 406. First and second S/Dcontact structures 132A and 132B having respective metal contacts 130Aand 130B and silicide layers 128A and 128B can be formed in contact withfirst and second S/D regions 110A and 110B, respectively. First S/Dcontact structure 132A can be connected to front-side power rails 105 byfirst interconnect structure 114 through first capping structure 136A.Second S/D contact structure 132B can be removed through opening 842 insecond capping structure 136B. After removal of the second S/D contactstructure, an air gap can form between the second capping structure andthe second S/D region. In some embodiments, both metal contact 130B ofsecond S/D contact structure 132B can be removed through opening 842. Insome embodiments, metal contact 130B and silicide layer 128B of secondS/D contact structure 132B can be removed through opening 842. In someembodiments, seal dielectric structure 142 can be formed in opening 842to seal air gap 126 between second capping structure 136B and second S/Dregion 110B. In some embodiments, seal dielectric layer 242 can beformed in air gap 226-2 during the formation of seal dielectricstructure 142. According to some embodiments, air gap 126 can reduce theparasitic capacitance between second S/D region 110B and gate structures112 of BPR semiconductor device 100. In some embodiments, the deviceperformance of BPR semiconductor device 100 can be improved by about 3%to about 5.5% due to capacitance reduction using air gap 126.

In some embodiments, a method includes forming a fin structure on afirst side of a substrate, forming a source/drain (S/D) region adjacentto the fin structure, forming a first S/D contact structure on the firstside of the substrate and in contact with the S/D region, and forming acapping structure on the first S/D contact structure. The methodsfurther includes removing a portion of the first S/D contact structurethrough the capping structure to form an air gap and forming a secondS/D contact structure on a second side of the substrate and in contactwith the S/D region. The second side is opposite to the first side.

In some embodiments, a method includes forming a fin structure on afirst side of a substrate, forming first and second source/drain (S/D)regions adjacent to opposite ends of the fin structure, and forming afirst S/D contact structure in contact with the first S/D region and asecond S/D contact structure in contact with the second S/D region onthe first side of the substrate. The method further includes removing aportion of the second contact structure through the second cappingstructure to form an air gap and forming a third contact structure on asecond side of the substrate and in contact with the second S/D region.The second side is opposite to the first side.

In some embodiments, a semiconductor device includes a fin structure ona first side of a substrate, first and second source/drain (S/D) regionsadjacent to opposite ends of the fin structure, a first S/D contactstructure in contact with the first S/D region, a first cappingstructure on the first contact structure, a second capping structureover the second S/D region, an air gap between the second cappingstructure and the second S/D region, and a second S/D contact structurein contact with the second S/D region on a second side of the substrate.The second side is opposite to the first side.

It is to be appreciated that the Detailed Description section, and notthe Abstract of the Disclosure section, is intended to be used tointerpret the claims. The Abstract of the Disclosure section may setforth one or more but not all possible embodiments of the presentdisclosure as contemplated by the inventor(s), and thus, are notintended to limit the subjoined claims in any way.

The foregoing disclosure outlines features of several embodiments sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art will appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art will also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method, comprising: forming a fin structure ona first side of a substrate; forming a source/drain (S/D) regionadjacent to the fin structure; forming a first S/D contact structure onthe first side of the substrate and in contact with the S/D region;forming a capping structure on the first S/D contact structure; removinga portion of the first S/D contact structure through the cappingstructure to form an air gap; and forming a second S/D contact structureon a second side of the substrate and in contact with the S/D region,wherein the second side is opposite to the first side.
 2. The method ofclaim 1, wherein the first S/D contact structure comprises a metalcontact, and wherein the removing the portion of the first S/D contactstructure comprises: forming an opening in the capping structure;removing the metal contact through the opening to form the air gap; andforming a seal structure in the opening to seal the air gap.
 3. Themethod of claim 2, wherein the forming the seal structure comprisesdepositing a dielectric material in the opening and in the air gap. 4.The method of claim 1, wherein the first S/D contact structure comprisesa metal contact and a silicide layer, and wherein the removing theportion of the S/D contact structure comprises: forming an opening inthe capping structure; removing the metal contact and the silicide layerthrough the opening to form the air gap; and forming a seal structure inthe opening to seal the air gap.
 5. The method of claim 1, wherein theforming the S/D region comprises: forming a first portion of the S/Dregion in the substrate; and forming a second portion of the S/D regionin contact with the fin structure, wherein the first S/D contactstructure is in contact with the second portion.
 6. The method of claim1, wherein the forming the second S/D contact structure comprises:replacing the substrate with a dielectric layer; removing a portion ofthe S/D region on the second side of the substrate; and forming asilicide layer in contact with the S/D region on the second side of thesubstrate.
 7. The method of claim 1, further comprising forming aninterconnect structure in contact with the second S/D contact structure.8. A method, comprising: forming a fin structure on a first side of asubstrate; forming first and second source/drain (S/D) regions adjacentto opposite ends of the fin structure; forming a first S/D contactstructure in contact with the first S/D region and a second S/D contactstructure in contact with the second S/D region on the first side of thesubstrate; forming a first capping structure on the first S/D contactstructure and a second capping structure on the second S/D contactstructure; removing a portion of the second contact structure throughthe second capping structure to form an air gap; and forming a thirdcontact structure on a second side of the substrate and in contact withthe second S/D region, wherein the second side is opposite to the firstside.
 9. The method of claim 8, wherein the second S/D contact structurecomprises a metal contact, and wherein the removing the portion of thesecond S/D contact structure comprises: forming an opening in the secondcapping structure; removing the metal contact through the opening toform the air gap; and forming a seal structure in the opening to sealthe air gap.
 10. The method of claim 9, wherein the forming the sealstructure comprises depositing a dielectric material in the opening andin the air gap.
 11. The method of claim 8, wherein the second S/Dcontact structure comprises a metal contact and a silicide layer, andwherein the removing the portion of the second S/D contact structurecomprises: forming an opening in the second capping structure; removingthe metal contact and the silicide layer through the opening to form theair gap; and forming a seal structure in the opening to seal the airgap.
 12. The method of claim 8, wherein the forming the second S/Dregion comprises: forming a first portion of the second S/D region inthe substrate; and forming a second portion of the second S/D region incontact with the fin structure, wherein the second S/D contact structureis in contact with the second portion.
 13. The method of claim 8,wherein the forming the third contact structure comprises: replacing thesubstrate with a dielectric layer; removing a portion of the second S/Dregion on the second side of the substrate; and forming a silicide layerin contact with the second S/D region on the second side of thesubstrate.
 14. The method of claim 8, further comprising: forming afirst interconnect structure on the first side in contact with the firstS/D contact structure; and forming a second interconnect structure onthe second side in contact with the third contact structure.
 15. Asemiconductor device, comprising: a fin structure on a first side of asubstrate; first and second source/drain (S/D) regions adjacent toopposite ends of the fin structure; a first S/D contact structure incontact with the first S/D region; a first capping structure on thefirst contact structure; a second capping structure over the second S/Dregion; an air gap between the second capping structure and the secondS/D region; and a second S/D contact structure in contact with thesecond S/D region on a second side of the substrate, wherein the secondside is opposite to the first side.
 16. The semiconductor device ofclaim 15, further comprising a silicide layer on the second S/D regionin the air gap.
 17. The semiconductor device of claim 15, furthercomprising a seal structure in the second capping structure to seal theair gap, wherein the seal structure comprises a dielectric material. 18.The semiconductor device of claim 17, further comprising a layer of thedielectric material in the air gap.
 19. The semiconductor device ofclaim 15, further comprising: a first interconnect structure on thefirst side of the substrate and in contact with the first S/D contactstructure; and a second interconnect structure on the second side of thesubstrate and in contact with the second contact structure.
 20. Thesemiconductor device of claim 15, wherein the first contact structurecomprises a metal contact and a first silicide layer, and wherein thesecond contact structure comprises a second silicide layer.